In semiconductor wafer processing, it is often required to process and/or test a wafer over temperature. Temperature-controlled wafer chucks have been developed to support a wafer and cycle the wafer over temperature during testing and/or processing. In a typical temperature-controlled wafer chuck, a temperature control module such as a heater and heat sink assembly is provided as a layer in the chuck. A top surface on which the wafer is mounted is provided at the top of the heater and heat sink assembly, and a base by which the chuck is mounted to the host apparatus, e.g., wafer prober, is provided beneath the heater and heat sink assembly. In some heater/heat sink assemblies, thermoelectric modules, also known as Peltier devices, are used to provide the temperature control. Peltier devices are small solid-state devices that function as heat pumps. The typical device is a sandwich formed by two ceramic plates with an array of small bismuth telluride (Bi2Te3) cubes in between. When a DC current is applied, heat is moved from one side of the device to the other, where it is removed with a heat sink. The cold side of the device is commonly used to cool a wafer. If the current is reversed, the device can be used as a heater by moving heat in the opposite direction.
One drawback to Peltier devices is that they are subject to mechanical stresses. For example, mechanical stresses induced by expansion and contraction of chuck layers over temperature can cause the devices to become unreliable and eventually fail. In some cases, the devices crack and are destroyed. This is particularly true in wafer chucks where the Peltier devices are rigidly attached to one or both of the upper and lower layers of the heater/heat sink assembly. As the assembly expands and contracts, the Peltier devices fail due to the mechanical stresses induced by thermal effects.
The present invention is directed to a workpiece chuck and method for supporting a workpiece such as a semiconductor wafer. The chuck includes a top layer on which the workpiece can be mounted and a temperature control assembly in thermal communication with the top layer to control temperature in the workpiece. The temperature control assembly includes an upper layer and a lower layer. At least one thermoelectric module is disposed between the upper and lower layers and at least one spacer is provided between the upper and lower layers. The spacer is sized to vertically space the upper and lower layers such that the thermoelectric module vertically floats in a space between the upper and lower layers. That is, the thermoelectric module is not mechanically or rigidly constrained within the temperature control assembly. As a result, mechanical stresses due to thermal effects are substantially reduced or eliminated.
In one embodiment, a thermally conductive medium is provided in the space between the upper and lower layers of the temperature control assembly. The thermally conductive medium thermally couples one or more thermoelectric modules to the upper and/or lower layers of the temperature control assembly. In one embodiment, the thermally conductive medium comprises a thermal heat sink grease. In another embodiment, the thermally conductive medium comprises a metallic foil. In another embodiment, the thermally conductive medium comprises a thermally conductive pad. In any of these embodiments, the thermally conductive medium is resilient such that the thermoelectric module is thermally coupled to the upper and/or lower layers while not being physically constrained between the upper and lower layers.
In one embodiment, the thermoelectric module is a Peltier device. The thermoelectric module can comprise bismuth telluride.
The upper and lower layers of the temperature control assembly can be fastened together by one or more screws. In one embodiment, the spacer is actually a plurality of washer or bushing-type spacers which include clearance holes through which the screws pass. As the screws are tightened to fasten the upper and lower layers they capture the spacers between the layers, while allowing sufficient space to prevent the thermoelectric modules from being vertically constrained. In another embodiment, the spacer is a unitary device, instead of a washer or bushing-type device as in the previously mentioned embodiment. The unitary device can be made in a star shape in which multiple spacing arms extend radially from the center of the temperature control assembly toward its outer edges.
The present invention provides numerous advantages over prior configurations. By providing sufficient space between the layers of the temperature control assembly, the thermoelectric modules are not subject to mechanical stresses due to thermal expansion and contraction effects. As a result, damage and failure of the devices is substantially reduced. Also, the spacers provide added rigidity and support in the wafer chuck, resulting in improved flatness of the top surface of the chuck, which in turn results in improved wafer processing and/or testing results.